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beat in rhythm to carry food from the water down to the mouth in the cap.
Crinoids range from the Ordovician Period to modern times, but they were most common and geologically most important in the Upper Palaeozoic. Beds of limestone are often dominated by crinoid fragments of this age, when ‘forests’ of crinoids must have covered large parts of the sea bed. The Silurian has been called the Age of Crinoids.
Recognising crinoids
Crinoids grow to a wide variety
of sizes. The stems commonly break up into the segments, called ossicles, and these disc-shaped pieces are scattered in many rocks. Each ossicle has a central hole.
It is much less common to find a head (calyx) and even rarer to find arms.
Calyx
Stem made from ossicles
Root
Floating crinoid using an air sac for buoyancy
Arms with cilia
Stalked crinoids
Radial plate
(Left) The calyx of a crinoid from the side, showing the plates with five-fold symmetry and connection to the stem.
(Right) The same calyx from above, showing the central mouth and five stubs where arms would have been attached.
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